THE PRODUCTION OF MINERALS
The different types of mineral products differ widely in their responsiveness to business conditions. At the one extreme anthracite coal shows hardly any influence of business conditions; at the other extreme the production of iron ore is one of the most sensitive indexes of industrial prosperity which we have. Copper and petroleum fluctuate widely but not in close harmony with other indexes of business activity; their ups and downs reflect to a large extent conditions peculiar to themselves. The responsiveness of an index of mineral conditions to business changes is therefore a matter of the relative weight given to different minerals. Indexes weighted by the value of the product in a base period, as are all those described in Chapter XX, reflect quite accurately the changes in trade shown by most other barometers, being dominated by soft coal, which shows a high correlation with general prosperity, but only a moderate range of fluctuation (aside from the influence of strikes).
Metals, on the whole, are more unstable in their rate of production than are the fuels; in fact constitute the most unstable of the major industrial groups.
The output of nearly every branch of metal mining is a raw material of industry and as such is sub